Grammarly AI Detector vs QuillBotAI Pro: An Honest Comparison (2026)

Full disclosure before anything else: I built one of the two tools in this comparison. You should read what follows with that in mind, and I've tried to write it so that the honest answer is sometimes "use the other one."

Grammarly's AI detector and QuillBotAI Pro both do the same nominal job and are both free to use without an account. They come from opposite directions, though, and that shapes almost everything about them. Grammarly is a writing assistant that added detection. We're a detector that doesn't write anything.

The short version: Grammarly fits better if you want to detect AI phrasing and then immediately fix it. QuillBotAI Pro fits better if you need to examine a result rather than act on it — reviewing someone else's work, checking a submission, or investigating a document where rewriting isn't the point.

What each one actually gives you

	Grammarly AI Detector	QuillBotAI Pro
Account required	No for the basic scan	No
Cost	Free (advanced features need Grammarly Pro)	Free, no paid tier
Word limit	None stated on the page	None
Output	Overall percentage plus flagged phrases	Sentence-level heatmap with per-sentence confidence
Models claimed	ChatGPT, Gemini, Claude, and Grammarly's own output	ChatGPT/GPT-4o/GPT-5, Claude, Gemini, Llama, Mistral
Rewriting built in	Yes — in-line rewrites via the Pro agent	No
Multilingual support	Not stated for detection	English, Urdu, Roman Urdu, Hindi, Spanish, French, German, Portuguese
Data retention	Governed by Grammarly's general privacy policy	Processed in volatile memory, not intentionally retained

Details for Grammarly are taken from their AI detector page as published.

Where Grammarly is genuinely the better choice

You're the author and you want to fix it. This is Grammarly's real advantage and it's not a small one. Their AI Detector agent flags phrases that read as AI-generated and offers in-line rewrites you can apply immediately. If your actual goal is "make this sound less like a machine wrote it," that's a complete workflow in one place, and we don't offer anything equivalent because we deliberately don't do rewriting.

You already live in Grammarly. If it's in your browser and your word processor, the marginal cost of using its detector is zero. Tool-switching friction is real, and "good enough and already open" beats "slightly better and requires a new tab" for most everyday checks.

You want one number. Sometimes a single percentage genuinely is what you need. Our heatmap is more information than some situations call for.

Where the design difference matters against them

Detecting their own output. Grammarly's page says it identifies AI text "from Grammarly and other tools." Reading a vendor's detector claims about that vendor's own generative features requires a certain amount of squinting — not because anyone's lying, but because the incentive structure isn't neutral. Ours isn't neutral either; I'd apply the same skepticism to our claims about our own tool.

Investigation rather than revision. If you're an educator or editor looking at someone else's work, the rewrite feature is irrelevant and the single percentage isn't enough. What you need is which sentences, and how confident the tool is about each. A document scoring 70% with flags scattered evenly reads very differently from one scoring 70% with flags concentrated in two paragraphs — the first often means the tool is penalizing writing style, the second suggests something was pasted in. A single number cannot distinguish those cases.

Non-native English writing. Grammarly's page doesn't state anything about detection behavior on multilingual or non-native English text. This is the failure mode with the most documented harm: a 2023 study in Patterns (Cell Press) found commercial detectors classified over 61% of supervised, human-written TOEFL essays as AI-generated while handling native-speaker essays near-perfectly. Any detector used on international students' work should say something about this. We built specifically for it and still don't claim immunity — no perplexity-based tool is immune.

Which should you use?

Use Grammarly if: you wrote the text yourself, you want to reduce AI-sounding phrasing, and you'd like the rewrite in the same window. Also if you already have it installed and want a quick sanity check.

Use QuillBotAI Pro if: you're evaluating text you didn't write, you need per-sentence detail to decide what a result means, you're working with non-native English writing, or you need the text not to be retained.

Use both if it's a decision that affects someone. Detectors disagree with each other constantly — different signal weightings, different thresholds, no shared benchmark. If two independent tools agree, that's mildly more informative than one. If they disagree, that itself tells you the text sits in ambiguous territory, which is worth knowing before you act.

And regardless of which you use: a detector score is a probability estimate, not evidence of authorship. That holds for both tools in this comparison.

Frequently asked questions

Is Grammarly's AI detector free?

The basic scan is free and doesn't require an account — you paste text and click Scan for AI. Their more advanced AI Detector agent, which offers in-line rewrites, is part of Grammarly Pro.

Which is more accurate, Grammarly or QuillBotAI Pro?

Neither of us can answer that credibly about our own tool, and I'd distrust any vendor who claims to. There's no shared public benchmark for AI detectors, and accuracy varies enormously by text type — length, editing history, and whether the writer is a native English speaker matter more than the brand. Test both on writing whose origin you already know.

Can either detector prove someone used AI?

No. Both produce statistical probability estimates, not evidence of authorship. Several universities, including Vanderbilt in 2023, have disabled AI detection tools partly on this basis.

Does QuillBotAI Pro rewrite text like Grammarly does?

No, deliberately. We're a detector only. If your goal is revising your own writing, Grammarly's integrated rewriting is a genuine advantage over us.

Do these tools store the text you paste?

QuillBotAI Pro processes input in volatile memory and does not intentionally retain it. Grammarly's handling is governed by their general privacy policy — worth reading directly if you're pasting confidential material into any tool.

Sources

Grammarly AI Detector — product page, features and claims as published.
Liang, W., Yuksekgonul, M., et al. (2023). GPT detectors are biased against non-native English writers. Patterns (Cell Press), Article 100779.

Other comparisons: GPTZero · Originality.ai · ZeroGPT · Turnitin alternatives. Or read how AI detection works before picking any of them.